Monitoring blood pressure and vascular parameters in small subjects such as companion animals, research animals, infants and preemies is an important diagnostic and research tool. Taking direct arterial blood pressure measurements can be painful, injurious, lead to further complications, and the direct method can only be acute in nature.
Non-invasive blood pressure measurement has become an essential part of modern medicine and research. Current methods generally involve a vascular occlusion cuff around an appendage and some method of detecting arterial pulsations. Methods for detecting arterial pulsations include photoelectric sensors, piezoelectric sensors, microphones, stethoscope, ultrasonic flow probes, secondary pneumatic cuffs or the occlusion cuff itself.
The procedure generally includes applying pressure to the appendage to compress the underlying arteries by inflating and pressurizing the arterial/venous vascular occlusion cuff until arterial blood flow ceases as determined by pulse termination from the pulse detector. Blood pressure readings are taken either while compression is being applied or while compression is being released. The pressure in the arterial/venous vascular occlusion cuff at the time of pulse termination or pulse initiation is universally accepted as the systolic blood pressure. Diastolic blood pressure is often taken as a return to nearly full amplitude of the pulse signal.
In smaller subjects, arterial pulsations are significantly smaller and of higher frequency. Subjects having 20 grams or smaller body weight such as mice have become the mainstay of modern medical research. These subjects have extremely small, high frequency pulsations that are almost impossible to detect with the above mentioned methods.
Venous occlusion plethysmography has been used as a method to access the health of peripheral vasculature in humans for decades. Generally, the technique involves placing an arterial/venous vascular occlusion cuff on the appendage proximal to a device that will measure the swelling of the appendage also known as a plethysmograph. The plethysmograph is a second, long pneumatic cuff, series of strain gages wrapped around the distal portion of the appendage, impedance or capacitance electrodes. The arterial/venous occlusion cuff is inflated to exert sufficient pressure to occlude the veins and stop the flow of venous blood out of the appendage. Blood flow into the appendage from the arteries keeps filling the vasculature which swells the appendage. The plethysmograph measures the swelling volume from which vascular parameters are determined.
Non-invasive blood pressure and vascular readings are also taken using arterial/venous occlusion plethysmography. This type of reading depends on the capacity of the appendage to swell or engorge during venous occlusion. Detection of the parameters in subjects with hypertensive or hyperemic appendages is difficult since the appendage is already supersanguinated and the surrounding vasculature is distended leaving little compliance for additional swelling.
Accordingly, it is an object of the present invention to provide a system and method for taking non-invasive blood pressure and other vascular parameter measurements in small subjects that overcomes the above-mentioned drawbacks and disadvantages.